1. Field of the Invention
This invention is related to the field of electronic control systems. More particularly, this invention is related to a control system of the type based on microprocessors, for controlling electromechanical devices. It is particularly useful as a magnetic bearing control system for a magnetic bearing arrangement, such as the type used to levitate a rotating shaft.
2. Description of the Problem
The problem of lubrication and wear in moving mechanical parts is as old as the utilization of mechanical devices. Various schemes have been devised to eliminate or reduce either or both of these problems with varying degrees of success. One way of alleviating these problems in rotating machines is to use magnetic bearings. Magnetic bearings are well known. A magnetic bearing allows a movable member (a rotor) of a machine to rotate freely with very little friction. This lack of friction is achieved by suspending the movable member, usually a shaft, within a housing lined with magnetic devices, so that the shaft can rotate without touching any solid surfaces. The shaft is suspended or levitated by magnetic fields.
FIG. 1 illustrates a cross-section of an example magnetic bearing. In this case the movable member consists primarily of a shaft, 110, which runs perpendicular to the paper. A disk, 109, made of laminated magnetic material is fixed to shaft 110. Four magnets, 101, 102, 103, and 104, are attached to a housing and distributed around the disk, 109. Electrical coils 105, 106, 107, and 108, are wound around the magnets and control the magnetic fields. In most cases, the magnet/coil combinations work in pairs. For example, magnets 101 and 103 work as a pair to control levitation of the shaft in the up/down direction in the drawing, and magnets 102 and 104 likewise work as a pair to control movement in the left/right direction. The housing for this bearing is not shown so that the details of the bearing itself can be shown more clearly.
FIG. 2 is a longitudinal section of a rotor being suspended by three magnetic bearings. Normally, the entire assembly is contained in a housing, which is not shown for clarity. Item 201 is a shaft, which is situated along axis 202. Laminated disk 205 is acted upon by bearing 204, which is shown in simplified form for clarity, but in reality includes an arrangement of magnets like that shown in FIG. 1, and sensors which detect the displacement of the shaft along the two control axes. Assuming a driving motor is positioned to the right in this illustration, bearing 204 is called an xe2x80x9cinboardxe2x80x9d radial bearing. Likewise, bearing 203 acts on disk 206. Again, assuming a driving motor located down axis 202 to the right, magnetic bearing 203 is called an xe2x80x9coutboardxe2x80x9d radial bearing. Housing 207 contains what is commonly known as a xe2x80x9cthrustxe2x80x9d bearing, and contains two electrically controlled magnets, 209 and 210, as well as an appropriate position sensor. These magnets act on disk 208 to control movement and position of the shaft along the axis 202 from left to right. The magnetic bearing system shown in FIG. 2 is an example only. It is possible to devise bearing systems of other shapes, which may have more or fewer bearings and more or fewer magnets in a given bearing. Other types of magnets may be used. In some machines, a thrust bearing may not be needed, for example, when a motor coupling provides axial support. In some applications, only a magnetic bearing on one end of a shaft is used, for example, if the other end is supported by other means. U.S. Pat. Nos. 5,216,308; 5,347,190; 5,543,673; and 5,986,373 provide background and additional information on this and other example magnetic bearing systems, and are incorporated herein by reference.
Generally, sophisticated electronics are required to vary the amount of field produced by the magnets in an electromechanical device such as a magnetic bearing. Control signals are produced for the magnets in response to position signals in order to maintain the rotor in levitation regardless of changing loads and/or mechanical conditions. The present commercial practice for active magnetic bearing control systems is a design in which each axis to be controlled, typically two orthogonal radial directions and one axial (thrust) direction, possesses an independent proportional-integral (PI), proportional-derivative (PD) or proportional-integral-derivative (PID) controller. However, electromechanical devices like magnetic bearings are difficult to control because they are inherently unstable, and so they have found only limited use in industry.
A well-known technique for controlling stable electromechanical systems is to employ the concept of a xe2x80x9cunified plant.xe2x80x9d As an example, consider the control system of FIG. 3. The control system, 302, consists of PID controllers, 303, and a compensator, 304. In this approach, a signal from the plant, P, 301, is passed through a matrix of digital filters in the compensator. The multidimensional filter undoes to some extent the transmission characteristics of the multi-dimensional plant. The filter operates on a vector of error signals measured at point 305. The filtering allows the controller gains to be increased theoretically without limit for an ideal stable plant with no time delay or other nonlinearity. The extent to which these gains can actually be increased is limited by how well the filter approximates the plant inverse and by any constraints on the power output.
The system above works well with stable plants. However, many practical electromechanical systems exhibit open-loop instability. Magnetic bearings, for example, exhibit a type of open-loop instability called xe2x80x9cnegative stiffness.xe2x80x9d Applying the unified plant approach to such a system is complicated due to the presence of this negative stiffness. To understand what is meant by negative stiffness, assume for the time being the absence of gravity. Allow a shaft to be balanced at its equilibrium position, as shown in FIG. 4. FIG. 4 shows a shaft, 401, and a magnetic bearing made up of four magnets, a left top (LT) magnet, 402, a right top (RT) magnet, 403, a left bottom (LB) magnet, 404, and a right bottom (RB) magnet, 405. If the bearing had positive stiffness, perturbations from equilibrium result in a restoring force that pulls the shaft back to equilibrium. The larger the perturbation, the stronger the restoring force. However, negative stiffness implies an unstable equilibrium. If the forces on the magnets are exactly balanced as shown in FIG. 4, any minute perturbation x causes a force F that grows with increasing distance from the equilibrium position until the shaft hits the stator (the stationary part of the magnetic bearing assembly). Magnetic bearings require a control force to overcome the effect of negative stiffness.
Some form of PI, PD or PID control is generally sufficient to overcome negative stiffness so that the shaft can be levitated. However, in order for the shaft to remain suspended under changing loads, the closed loop positive stiffness, and hence the feedback gains must be large. The resistance of the bearing to motion caused by changing loads is referred to as its xe2x80x9cdynamics stiffnessxe2x80x9d. In uncompensated systems, the large feedback gains required for dynamic stiffness cause stability problems related to plant dynamics other than negative stiffness. This will be the case when nonlinear dynamic characteristics (e.g., time-delay) and when cross-response structural resonances are present in the sensor bandwidth. Due to the above-described instability problems, current magnetic bearing control systems have major drawbacks. Such systems exhibit stability sensitivity and relatively narrow controller bandwidth in each direction. There are alternative approaches for improving dynamic stiffness that involve deriving an adequate, low-order model from measurements or simulation data. These are referred to as state-space models. This derivation is not straightforward, and can require significant off-line time by a very skilled practitioner in the control theory and system identification fields. Automated design algorithms are often intractable for complex, highly dynamic, electromechanical systems. Therefore, control systems based on state-space models are difficult to design and expensive to use in commercial applications. What is needed is a new type of control system that can handle open-loop instability such as negative stiffness, but that can also be based on a unified plant so that the control system is more straightforward to adapt to various electromechanical arrangements.
The present invention solves the above problems by providing a stand-alone, wide-bandwidth control system without the drawbacks of inadequate compensation, cross-coupling sensitivity, and lack of resonance control. A high performance multi-dimensional control system is realized with higher feedback gains, without the attendant threat of instability from uncompensated dynamics. For magnetic bearings, the control system of the invention offers high magnetic bearing stiffness over a wide bandwidth. Implementing this new architecture on a real-time processing platform as a stand-alone system can make the control system of the invention attractive for practical commercial implementation. The control system is particularly useful for magnetic bearings but can be adapted to any. electromechanical device or arrangement that exhibits open-loop instability.
A magnetic bearing system that makes use of the invention includes one or more magnetic bearings operable to suspend a movable member in response to control signals. In one embodiment, sensors are co-located with the bearings to detect displacement of the movable member. The sensors provide sensing signals. A control unit within the control system for the bearings processes the sensing signals. The control unit according to one embodiment of the invention provides, for each bearing/sensor direction, a control signal or an output signal so that the magnetic bearing maintains the movable member in the desired position. The control unit according to one embodiment of the invention includes a specially designed compensation filter, which isolates the negative stiffness by removing substantially all plant dynamics except the negative stiffness characteristic so that the movable member is treated as a pure mass, thus providing for better stiffness, improved bandwidth, and other improved characteristics. The control unit output signal or signals may be the control signal or signals that are fed to a magnetic bearing, or the control unit output signals may be a portion of the control signals or may need to be amplified or otherwise processed to produce the actual control signals.
The control unit according to one embodiment of the invention has inputs for receiving sensing signals, a set-point signal, and a tachometer signal. The control unit outputs an output signal or a control signal for each axis to be controlled. In one embodiment, the control unit includes a resonance controller for adaptively filtering a negated sensing signal and producing a resonance controller output signal from the filter. A summer is connected to the resonance controller, and adds the set point signal and the negated sensing signal with the resonance controller output signal to produce an error signal. A compensation filter, sometimes called, xe2x80x9ca compensatorxe2x80x9d, is also connected to the first summer for processing the error signal and producing a compensator output signal. The multi-dimensional compensation filter isolates open-loop instability (negative stiffness in the case of magnetic bearings) so that the movable member is treated as a pure mass. The control unit also includes a band-shaping filter. The control unit in one embodiment also includes a proportional-integral-derivative (PID), proportional-integral (PI), or a proportional-derivative (PD) vector to control signals with a PID, PI, or PD controller, respectively. At some places in this disclosure we use the term xe2x80x9cPID vectorxe2x80x9d or xe2x80x9cPID controllerxe2x80x9d to refer to an element that can be any of these. In many cases, the movable member is a shaft that rotates. The PID controller produces all or a portion of the control unit output signal. A narrowband controller connected to the sensing signal and the tachometer signal input provides a signal that compensates for imbalance forces that occur when the shaft rotates. In this case, another summer adds the PID controller output signal and the narrowband controller output signal to produce the control unit output signal. Instead of a separate narrowband controller to cancel imbalance forces due to shaft rotation, a tracking notch filter is sometimes used to remove those narrowband components from the control signal. The notch filter would typically filter the digitized sensing signal with the frequency of the notch determined by the tachometer signal.
The various filters, summers, and other operators required to carry out the invention are preferably implemented on a programmed processing platform such as a digital signal processor (DSP) or an arrangement of multiple digital signal processors. Such an implementation makes it possible to execute any software or microcode necessary to carry out the high-speed filtering required by the compensator of the invention. The construction of the compensator is typically done off-line, that is by way of background as opposed to real-time processing. Constructing compensators that adjust for changes in the magnetic bearing dynamics requires real-time system identification, as well as processing of the compensation algorithms even though these processes occur at a slower rate than the control processing. The real-time and background processing software or microcode in combination with the processing hardware forms the means to implement the invention. Such a system can be used to implement only the control unit, or it can be used to implement all aspects of the control system.